This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. In children, auditory processing disorder (APD) presents as difficulty processing speech despite audiometrically normal hearing. This difficulty is most pronounced in the presence of competing background noise, which represents most typical real-world listening situations. Anatomical and functional observations from the cochlea up to the cortex favor a right ear advantage in normal individuals. In almost all right handed and most left handed people, speech is also predominantly processed in the left cerebral hemisphere. The asymmetrical features of both afferent and efferent auditory pathways suggest that competing signals from both ears are processed with a right ear advantage which enables the left hemisphere to process speech appropriately in difficult listening situations. Our overall hypothesis is that the efferent system is disrupted in children with APD, resulting in a system that performs poorly when stimuli are presented in noise, and fails to demonstrate the right ear advantage. In the first aim, we will examine ear advantages and interhemispheric asymmetry in APD using behavioral (dichotic listening paradigm) and electrophysiological measures (auditory event related potentials) in children with APD vs. age-matched controls in quiet and in noise conditions. In the second aim, we will determine the potential asymmetrical features of the cochlear active mechanism and efferent pathways in children with APD vs. control subjects using otoacoustic emissions and their suppression by noise. In the third aim, we will measure speech processing abilities in different conditions of noise to activate part of or, the entire efferent system. We will correlate the effects of the efferent system activation on speech abilities with the effect of the efferent system activation on the cochlear active mechanism, in children with APD vs. normal children. Together, these studies will provide us with a unique view of afferent and efferent functioning in APD and may guide us to novel treatment strategies for children with APD.